A diesel engine lubricated with light oil is described in Japanese Utility Model Patent Application Publication No. 60-194112. This diesel engine has a reserver tank for bubble separation, which is disposed between a fuel tank and a diesel engine body. A lubrication-system fuel recycling circuit and a combustion-system fuel recycling circuit are provided between the reserver tank and the engine body. This type of fuel lubrication diesel engine uses light oil as not only fuel but also lubricating oil, and light oil lubricates parts of the engine body. Thus, there is no need for oil exclusive used for lubrication, and the trouble of oil exchange can be saved.
FIG. 11 schematically illustrates a fuel lubrication diesel engine 100 as described in the above-mentioned application, and mainly shows fuel supply systems thereof. The diesel engine 100 is equipped with a fuel tank 101, a reserver tank 102 having a combined function of a bubble separator, and an oil pan 103. The fuel tank 101 and the reserver tank 102 are connected via an oil passage composed of a water-oil separator (sedimenta) 104, and a supply pump (oil supplying pump) 105.
The diesel engine 100 has a lubrication-system fuel recycling circuit 108 that supplies fuel serving as lubricating oil to engine parts 107 necessary for supply of lubricating oil. The lubrication-system fuel recycling circuit 108 has a cooling device 112, a filter 109 and a lubricating oil pump 110. The lubricating oil pump 110 is driven to pump up fuel from the reserver tank 102 and supply it to the engine parts 107. The fuel that has lubricated the engine parts 107 flows down the engine body to the oil pan 103. Then, the fuel is pumped up by a scavenging pump 111 and is returned to the reserver tank 102.
The diesel engine 100 of fuel lubrication type has a fuel-system fuel recycling circuit 113, which supplies fuel to an injection system 114 that injects fuel into cylinders. The fuel-system fuel recycling circuit 113 has the cooling device 112 shared by the lubrication-system fuel recycling circuit 108, a filter 115 and an injection pump 116. The injection pump 116 is driven to pump up fuel from the reserver tank 102 and supply it to the injection system 114. Fuel that is supplied to the injection system 114 but is not used for combustion is returned to the reserver tank 102.
As described above, the diesel engine 100 of fuel lubrication type described in the above-mentioned application has two fuel recycling circuits, namely, the lubrication-system fuel recycling circuit 108 and the fuel-system fuel recycling circuit 113.
However, the diesel engine 100 has three tanks, which are the fuel tank 101, the reserver tank 102, and the oil pan 103. Further, the diesel engine 100 has four pumps, that is, the supply pump 105, the lubricating oil pump 110, the injection pump 116 and the scavenging pump 111. Thus, the diesel engine 100 has a very complicated structure.
The diesel engine 100 has another problem. As described above, the fuel pumped up from the reserver tank 102 is supplied to the shared cooling device 112, and is then branched toward the lubricating oil pump 110 and the injection pump 116. Here, it should be noted that the lubricating oil pump 110 and the injection pump 116 have quite different discharge capacities and discharge pressures. When fuel is supplied to these pumps via the common supply port, the injection performance and the lubrication performance may be mutually affected.